An ordered three-dimensional (3D) microstructure is an ordered 3D structure at the micrometer scale. Currently, polymer cellular materials that are mass produced are created through various foaming processes, which all yield random (not ordered) 3D microstructures. Techniques do exist to create polymer materials with ordered 3D microstructures, such as stereolithography techniques; however, these techniques rely on a bottom-up, layer-by-layer approach which prohibits scalability.
A stereolithography technique is a technique that builds a 3D structure in a layer-by-layer process. This process usually involves a platform (substrate) that is lowered into a photo-monomer (photopolymer) bath in discrete steps. At each step, a laser is scanned over the area of the photo-monomer that is to be cured (polymerized) for that particular layer. Once the layer is cured, the platform is lowered a specific amount (determined by the processing parameters and desired feature/surface resolution) and the process is repeated until the full 3D structure is created. One example of such a stereolithography technique is disclosed in Hull et al., “Apparatus For Production Of Three-Dimensional Objects By Stereolithography,” U.S. Pat. No. 4,575,330, Mar. 11, 1986, which is incorporated by reference herein in its entirety.
Modifications to the above described stereolithography technique have been developed to improve the resolution with laser optics and special resin formulations, as well as modifications to decreasing the fabrication time of the 3D structure by using a dynamic pattern generator to cure an entire layer at once. One example of such a modification is disclosed in Bertsch et al., “Microstereolithography: A Review,” Materials Research Society Symposium Proceedings, Vol. 758, 2003, which is incorporated by reference herein in its entirety. A fairly recent advancement to the standard stereolithography technique includes a two-photon polymerization process as disclosed in Sun et al., “Two-Photon Polymerization And 3D Lithographic Microfabrication,” APS, Vol. 170, 2004, which is incorporated by reference herein in its entirety. However, this advance process still relies on a complicated and time consuming layer-by-layer approach.
3D ordered polymer cellular structures have also been created using optical interference pattern techniques, also called holographic lithography; however, structures made using these techniques have an ordered structure at the nanometer scale and the structures are limited to the possible interference patterns, as described in Campbell et al., “Fabrication Of Photonic Crystals For The Visible Spectrum By Holographic Lithography,” NATURE, Vol. 404, Mar. 2, 2000, which is incorporated by reference herein in its entirety.
Previous work has also been done on creating polymer optical waveguides. A polymer optical waveguide can be formed in certain photopolymers that undergo a refractive index change during the polymerization process. If a monomer that is photo-sensitive is exposed to light (typically UV) under the right conditions, the initial area of polymerization, such as a small circular area, will “trap” the light and guide it to the tip of the polymerized region due to this index of refraction change, further advancing that polymerized region. This process will continue, leading to the formation of a waveguide structure with approximately the same cross-sectional dimensions along its entire length. The existing techniques to create polymer optical waveguides have only allowed one or a few waveguides to be formed and these techniques have not been used to create a self-supporting three-dimensional structure by patterning polymer optical waveguides.
As such, there continues to be a need to create polymer cellular materials with ordered microstructures on a large and useful scale using a simple technique.